When a metal powder sheet formed by rolling a mixture of the metal powder and a binder is placed on a metal base material and sintered for forming the metal layer thereon, the binder in the metal powder sheet, even if the binder has originally an adhesive action, is burned and volatilized off in the temperature raising stage for sintering to lose the adhesive function, whereby the adhesion between the metal powder sheet and the base material is lost. Thus, when the weight of the metal powder sheet acts on the adhesion face with the base material as the case of bonding the metal powder sheet to a slanting surface or a curved surface of the base material or a downward surface of the base material, the adhesive force of the binder cannot support the weight of the metal powder sheet, whereby the metal powder sheet peels off or falls from the base material. In particular, in the case of performing the sintering treatment of the metal powder sheet, which suffers vibration or impact during conveying in a furnace such as a mesh-belt type or pusher type continuous sintering furnace, a vacuum sintering furnace, etc., the metal powder sheet tends to peel off or fall due to the vibration or impact.
As an effective method for overcoming the above-described problem, the inventors already proposed a method of preventing the occurrence of peeling off or falling of the metal powder sheet even in the case of bonding the sheet to a metal base material in the specific state as described above and performing the sintering treatment as applying vibration or impact to the metal powder sheet by interposing a specific composition between the metal powder sheet and the metal base material in the case of disposing the sheet on the base material to assist the adhesion or fixing of the sheet to the base material until the metal powder sheet is sintered (Japanese Patent Application No. 34887/84, corresponding to U.S. Pat. application Ser. No. 705,264, filed Feb. 25, 1985).
However, since the composition used in the previously proposed method is mainly composed of a (meth)acrylic acid alkyl ester polymer, a specific sintering operation that the sintering system is maintained at 250.degree. to 380.degree. C. for a definite period of time in the initial stage of temperature raising is required and the desired effect cannot be attained without employing such a specific operation. Such a sintering operation is not always practical and is somewhat lacking in general industrial utilization.
Also, when a metal powder sheet is placed on a metal base material and sintered, there is a problem that this kind of sheet shrinks about 20% in the plane directions at sintering and hence it is usually necessary to employ the dimensions of the metal powder sheet estimating the shrinkage so that the definite dimensions can be obtained after sintering. However, such a selection of dimensions of the sheet is very troublesome and is lacking in practicality. Further, the occurrence of deviation of shrinkage percentage by the scattering in the thickness of the metal powder sheets or the mixing ratio of a metal powder and a binder is unavoidable, which causes a difficulty that it is difficult to make the dimensions of the sheet after sintering in the definite dimensions. This difficulty becomes more serious for a larger size of the sheet since the deviation of the shrinkage percentage becomes larger.
Accordingly, it is very convenient if the occurrence of such a shrinkage of the sheet can be restrained by interposing a composition as described above but such a shrinkage restricting function cannot be expected by the above-described composition.